feat(agent): wire up agentssh server to allow exec into container (#16638)

Builds on top of https://github.com/coder/coder/pull/16623/ and wires up
the ReconnectingPTY server. This does nothing to wire up the web
terminal yet but the added test demonstrates the functionality working.

Other changes:
* Refactors and moves the `SystemEnvInfo` interface to the
`agent/usershell` package to address follow-up from
https://github.com/coder/coder/pull/16623#discussion_r1967580249
* Marks `usershellinfo.Get` as deprecated. Consumers should use the
`EnvInfoer` interface instead.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>
Co-authored-by: Danny Kopping <danny@coder.com>

agent/usershell/usershell_other.go

@@ -11,6 +11,7 @@ import (
 )
 
 // Get returns the /etc/passwd entry for the username provided.
+// Deprecated: use SystemEnvInfo.UserShell instead.
 func Get(username string) (string, error) {
 	contents, err := os.ReadFile("/etc/passwd")
 	if err != nil {